The effort to provide more reliable, more cost-effective means for interconnecting microelectronic devices has been underway for at least thirty years. The need for such improvement has become steadily greater because of the long-term trend toward greater circuit density, and the consequent need to reduce the size of bonding pads on a circuit chip, and to narrow the spacing between pads.
The majority of semiconductor devices are assembled with the use of wire bonding to provide ohmic interconnections to the metal pads on the circuit chip. Currently available wire bonders require at least 2 mils spacing from the center of one pad to the center of an adjacent pad. This limitation may block further reductions in the size of a circuit chip, and further increases in circuit density. This and other limitations of wire bonding have been known for several decades; and yet, there is not apparent hope for a break through in wire bonding that would accommodate dramatic reductions in the size and spacing of bonding pads. Patents for anisotropically conductive adhesives extend back to Jun. 5, 1956 with W. Barrows Electrically Conductive Coating, U.S. Pat. No. 2,748,701. Activity in the area of anisotropically conductive adhesives increased in the 1980's with the development of liquid crystal displays and the needs associated for electrical connection of these devices to the support circuitry that make them function. More patents for Z-axis adhesives and materials exist than actual commercially available products (see attached list). The Z-axis materials that are commercially available use either thermoplastic adhesives, unreacted thermosets, or partially reacted thermoset adhesives as the insulating media to separate the metallic conductors, but do not combine these materials into discreet layers. The density of the interconnects to be made are a function of the size of the metallic conductors used in the adhesive and the ability to keep these conductors isolated from one another.
U.S. application Ser. No. 08/719,052 filed Sep. 24, 1996 discloses the interconnection of microelectronic parts by means of multiple, discrete, conductive nanoscopic fibrils or tubules fixed within the pores of an insulating film.